1. Field of the Invention
The present invention pertains to seismic data acquisition and processing, and, more particularly, to a method and apparatus for attenuating wind noise in seismic data.
2. Description of the Related Art
In a seismic survey, an acoustic or other seismic signal is imparted into a subterranean geological formation. As the signal propagates through the formation, it is reflected back to the surface. Seismic sensors (e.g., geophones, in a land survey) are positioned on the surface to receive the reflections. The reflections are received and converted to data representative of the formation's characteristics and structure. The data is then analyzed to detect the presence of hydrocarbons or other fluids and minerals of interest. Because the cost of extracting such deposits is high, accurate data yielding correct results is highly desired.
Seismic surveys are typically performed in uncontrolled environments. Wind is a common phenomenon with seismic surveys leaving the data degraded with broadband noise. The efficient attenuation of wind noise is therefore of major importance for high quality seismic surveys. Studies of the wind noise have revealed that it has a locally coherent structure. However, when employing vibroseis (i.e., a vibrator powered by a sweep signal) as the seismic source, the data conventionally is correlated without any prior noise attenuation. The internal coherence of the wind noise is therefore destroyed, leaving a pseudo-random noise in the seismic data, which hardly can be removed in processing. More technically, the correlation is performed by a convolution between the survey signal and the resultant seismic data, and the wind noise cannot then be retrieved through deconvolution.
More particularly, wind is a common feature in surface seismic acquisition, especially in desert environments. In general wind is defined by a turbulent flow of air with a turbulent velocity profile above the ground. This means that the average wind velocity increases logarithmically from the ground towards the free air flow.
Observations in the field show that the turbulent flow of air is not a straight movement in layers parallel to the ground. Rather, wind consists of an ensemble of turbulent patches, which move in the average direction of the wind. Within the turbulence patches, the wind imposes a coherent downward pressure on the ground, which in turn is converted into a surface wave. These wind induced, locally coherent, surface waves represent a substantial fraction of wind noise and can not be avoided by burying the geophone.
Conventional seismic recording using vibroseis as a source employs correlation of the geophone with the pilot sweep straight after acquisition. No processing is carried out prior to correlation. The correlation with the pilot sweep, however, has a significant impact on the wind noise. Since wind noise and seismic signal are not correlated, the local phase and amplitude consistency of the wind noise is destroyed. Hence after correlation with the pilot sweep no wind noise filtering based on wind noise coherence can be carried. This implies that all wind noise filtering will result in degradation of the seismic signal.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.